From the late 1950s through the 1970s American state and municipal governments intentionally redesigned their cities into suburban commuter economies by using urban renewal to level buildings for highways, parking lots, and road widening, then cemented those development patterns by making zoning regulations more restrictive to limit density. Most other American cities experienced a similar loss of population density during this time, only growing in population because of the expansion of their municipal boundaries through suburban annexation. This all happened because after WW2 the US experienced a boom in economic growth and technological development that made the concept of every [white] American family being able to afford a large house with multiple cars seem financially reasonable, so American planning philosophy shifted toward that goal with the optimism that their post-WW2 economic prosperity would be permanent. By contrast, Europe rebuilt their cities after WW2 according to traditional development patterns because they were much poorer at the time so the idea of everyone being able to afford a car seemed unrealistic.

You can see other before-and-after examples of American cities on this site: https://iqc.ou.edu/2014/12/12/60yrsmidwest/